Linear and other types of servomechanisms have long been known for use in machine control arts. Many existing methods of servo control, however, involve expensive, large, and intricate apparatus, and are not generally considered useful in electronic probing or dexterous module handling frequently encountered in modern manufacturing environments. Modern manufacturing facilities frequently employ robot manipulators having end-of-arm tools such as grippers or fingers for manipulating small parts, and these applications require small and accurate actuators and sensors.
Electric servo systems typically employ rotary actuators (motors) which require a complex and error-prone transmission system to convert the rotary motion into a linear motion frequently desirable for assembly and test fixtures. Additionally, the requisite position and velocity sensors and motor power supplies often make electric servo systems prohibitively expensive and bulky for many applications.
Direct-current stepping motors are also sometimes used in servomechanisms, but these devices suffer from low power and also require some form of mechanical transmission to create a linear motion. Stepping motors are open loop devices which are susceptible to error unless used in conjunction with a rotary position sensor for feedback.
One device known for positioning control of a linear actuator includes a pneumatic cylinder coupled by a ball screw to a rotary encoder, which detects the position of the actuator rod and provides a position signal to a control unit. The control unit typically counts transmitted pulse signals from the encoder, and when the actuator rod reaches a preset position, the valve driving the cylinder is closed and the control unit actuates a magnetic brake to halt the movement of the actuator rod. This device is expensive and complex due to the high cost of incorporating a rotary encoder, a ball screw driving mechanism for the encoder, and a magnetic brake.
Another approach to employing fluid linear actuators in servomechanisms involves use of a magnetic piston in the cylinder to actuate magnetic reed switches or a Hall effect sensor. Typically, however, this approach is used to provide an end-of-stroke signal to a programmable controller to indicate that the cylinder is not jammed. The position of the reed or Hall-effect sensor can be manually adjusted along the stroke of the cylinder so that a signal may be generated at various actuator positions. The signal can also be used to open and close solenoid valves for crude mid-stroke positioning of the actuator. However, the positional inconsistency of the reed switch actuation, the lack of controlled deceleration, and the inherent compliance or "springiness" of the air cylinder make the system highly inaccurate. Other drawbacks to the magnetic switch method include the requirement for manual movement of the switches to change the stopping position of the actuator, and the limitation on the number of switches that can be employed on one cyliner due to the relatively large package size of the switch body.
Linear variable differential transformers (LVDT) are also known for measuring linear displacement, but these devices typically involve precision winding on the secondary coils so as to provide a transformer secondary output which varies linearly with the movement of a magnetic plunger which moves axially within the primary and secondary coils. These devices are expensive and do not lend themselves readily to incorporation with fluid linear actuators due to the difficulties in mid-stroke position control of such types of actuators.
Hydraulic servo systems often use precision machined servo valves which must be individually flow tested. The high degree of precision required in manufacturing together with the relatively low volumes of production by current vendors lends to high costs for hydraulic servo mechanisms.
One known approach for actuator control in closed-loop fluid servo systems is the electrohydraulic, variable-displacement axial-piston pump. This pump provides an output flow in proportion to the level of a DC analog command signal from a programmable controller. This command signal is applied to a pulse-width-modulating control unit which compares a command signal (commanded pump displacement) to an analog feedback signal (measured actual pump displacement) from a potentiometer coupled to the pump yoke. The error signal is amplified and converted to a pulse-width-modulated output that controls pump yoke movement through a proportional valve. The amplitude (voltage) of these pulses is fixed, but the width (duration) is proportional to the difference between commanded and actual displacement. The error signal is applied through a power stage to one of two solenoids of a three-way proportional valve, which controls both the direction and rate of flow routed to the pump's yoke mechanism. While this device has been proven successful to minimize overshoot, the proportional valves employed are precision-machined and are consequently very expensive. Such an approach is also generally large and cumbersome and unsuited for use with small-scale systems.
Accordingly, there is a need for a low cost linear servoactuator system which does not involve the use of rotary encoders, expensive precision-wound LVDT's, precision servo or proportional valves, or other expensive or complex components. The advent of the low-cost microcomputer has made possible the incorporation of low-cost devices and components whose performance may be optimized by monitoring and controlling operation thousands of times per second. There are now available low-cost solenoid fluid valves which have extremely rapid response times which can be employed for control of fluid actuators. Additionally, there are now available low-cost analog-to-digital converters which can be employed to convert analog signals from sensors for use by microcomputers. Accordingly, a successful approach to solving the problem of providing low-cost linear servoactuator control will represent the merger of analog electronics, digital electronics, and fluid mechanics.